An existing device that reads a signal from an information recording medium such as a hard disc device or an optical disc device detects an information signal binarized through signal processing such as wave form equalization under the assumption that the signal read from the information recording medium has a substantially linear characteristic.
In recent years, following improvement in recording density of the information recording medium, interference between recorded signals ahead and behind has been unable to be disregarded. Thus, a method, in which a readout signal is equalized to multiple levels by using principles of partial response (PR) and a maximum likelihood detection by using a Viterbi detection method is further performed, has been in increasingly wide spread use.
FIG. 1 shows an example of a configuration of such an information readout device. This information readout device 80 includes: a PLL circuit 82, a transversal filter 84, and a Viterbi detector 86. For a signal read out from a recording medium, a clock is extracted at the PLL circuit 82, and a readout signal is equalized to a desired PR class by the transversal filter 84 that performs linear equalization. For example, in a case of PR121, the readout signal is equalized to five multiple levels. The Viterbi detector 86 performs the maximum likelihood detection on the equalization signal outputted from the transversal filter to read out a so-called probable binary signal group which is the closest to a signal group of an ideal waveform in consecutive equalization signal groups.
Where a tap length is n, the transversal filter can be expressed as follows.
                              y          ⁡                      (            k            )                          =                              ∑                          i              =              0                                      n              -              1                                ⁢                                                    h                1                            ⁡                              (                i                )                                      ·                          x              ⁡                              (                                  k                  -                  i                                )                                                                        (        1        )            In formula (1), y (k) is an output signal of the filter at a time k, x (k) is an input signal at the time k, and h1 (i) is a tap coefficient (i=1, 2, 3, n−1). In a typical information readout device, a value of approximately 5 to 10 is used as the tap length. As clear from the formula (1), an equalization characteristic is a linear characteristic in any tap length.
However, in a case where equalization to multiple levels corresponding to a target PR waveform is performed by a linear equalization filter such as a transversal filter, compared to simple equalization to binary values conventionally practiced, a nonlinear characteristic possessed by a readout signal becomes more influential. This is partially attributable to that a linear characteristic in an amplitude direction becomes more important since there are plural threshold values for judging a signal level in a case of the equalization to multiple values whereas there is one such threshold value in a case of the equalization to binary values.
Thus, as one of methods of equalizing a signal having such a nonlinear characteristic, a method using a filter called Volterra filter expressed by a polynomial is suggested. For example, Japanese Laid-Open Patent Application JP-P 2005-303361A, JP-P 2006-313592A, and JP-P 2005-71565A discloses a signal processing device that a Volterra filter equalizes a readout signal read from a recording medium.
As an order of this Volterra filter, a third order and a fourth order are theoretically possible. However, considering tap coefficient stability and circuit complication, a second-order filter can be said to be practical. However, compared to a conventional filter with a linear characteristic only, circuit complication is inevitable. For example, where a tap length of a first-order term of the filter is n, the number of tap coefficients is n, but when a tap length of a second-order term is n, the number of tap coefficients is n×n. In practice, the number of independent tap coefficients is n×(n+1)/2 based on symmetry of diagonal components, but it is clear that more processing is required for the number of the second-order terms than for the number of the first-order term. Therefore, a circuit size, a tap coefficient setting stability, etc. become practical problems.